A loudspeaker is an electromechanical transducer that converts electrical signals to sound. Such an electromechanical transducer is also called as a driver or a woofer. The term loudspeaker has also been used of loudspeaker systems which comprise one or more electromechanical transducers, an enclosure (a housing), and optionally additional electronics. In this application the term woofer is mainly used to refer to the electromechanical transducer which comprises inter alia a coil or some other element which converts an electric signal to a mechanical force, and a sound producing unit (usually called as a cone) which is affected by the mechanical force to produce sound on the basis of the electric signal. It should be noted here that the term cone is not restricted to cone-like membranes but also other physical appearances of the sound producing unit can be implemented with the present invention.
The international patent application WO 2004/082330 discloses a woofer equipped with measurement of the movement of the cone of the woofer unit. The measurement is based on detecting a change in a measurement capacitance. The measurement capacitance is formed by adding a cylindrical, conducting plate around the vibrating coil of the woofer. When the coil and the membrane attached with the coil move, the capacitance of the measurement capacitor changes. This change of the capacitance is measured to determine how far the coil and the membrane have moved from the rest position.
The European patent application EP 0 213 319 discloses a woofer having a membrane coupling arrangement to measure the movement of membrane of the woofer. The arrangement comprises a sensor unit and a metal plate. The metal plate is fixed to the membrane of the woofer. The sensor unit is positioned near the membrane of the woofer. Hence, the sensor unit senses the movements of the metal plate when the membrane vibrates along with audio signals.
Both of these system require that the woofer is modified by adding either a conducting plate e.g. a metal plate to the membrane or to the coil of the woofer.
The Chinese patent application publication CN 1173105 discloses a pseudo-zero impedance loudspeaking technique and a sound system. The system uses the feature that the physical quantities relative to the moving speed of vibration membrane in woofer are real-time measured and the physical quantities along with audio signals picked up by sound system can control the movements of vibration membrane. The measurement of the movements of the membrane is based on measuring the changes of the impedance in the woofer driving circuit. The measurement is not based on the real movements of the membrane.